Enhancing Nitrogen Reduction Reaction through Formation of 2D/2D Hybrid Heterostructures of MoS2@rGO

Given the challenging task of constructing an efficient nitrogen reduction reaction (NRR) electrocatalyst with enhanced ambient condition performance, properties such as high specific surface area, fast electron transfer, and design of the catalyst surface constitute a group of key factors to be taken into consideration to guarantee outstanding catalytic performance and durability. Thereof, this work investigates the contribution of the 2D/2D heterojunction interface between MoS2 and reduced graphene oxide (rGO) on the electrocatalytic synthesis of NH3 in an alkaline media. The results revealed remarkable NRR performance on the MoS2@rGO 2D/2D hybrid electrocatalyst, characterized by a high NRR sensitivity (faradaic efficiency) of 34.7% with an NH3 yield rate of 3.98 ± 0.19 mg h–1 cm–2 at an overpotential of −0.3 V vs RHE in 0.1 M KOH solution. The hybrid electrocatalysts also exhibited selectivity for NH3 synthesis against the production of the hydrazine (N2H4) byproduct, hindrance of the competitive hydrogen evolution reaction (HER), and good durability over an operation period of 8 h. In hindsight, the study presented a low-cost and highly efficient catalyst design for achieving enhanced ammonia synthesis in alkaline media via the formation of defect-rich ultrathin MoS2@rGO nanostructures, consisting predominantly of an HER-hindering hexagonal 2H-MoS2 phase.


INTRODUCTION
Clean and sustainable ammonia (NH 3 ) is considered an ideal carbon-free energy carrier useful for mitigating issues related to environmental pollution as well as drastic consumption of nonrenewable fossil fuels during the production of inorganic chemicals for the agricultural, medical, or pharmaceutical industries. 1,2In industry, NH 3 production depends predominantly on the century-old Haber−Bosch process that operates at high temperature and pressure (∼500 °C, ∼25 MPa) over iron (Fe)-and/or ruthenium (Ru)-based catalysts, consuming ∼2% of the global energy and subsequently generating up to 2 tons of yearly worldwide greenhouse gas emissions. 3For this reason, it is strikingly important that the development of greener, more energy-efficient, and sustainable routes to produce NH 3 is investigated and employed.To date, various methods such as the biological method using purified nitrogenase bacteria 4 and the photocatalysis 5 and electrocatalysis 6,7 routes have been explored to promote the reduction of dinitrogen (N 2 ) to NH 3 at ambient conditions.Among these reduction routes, the electrocatalytic nitrogen reduction reaction (NRR, N 2 + 6H + + 6e − →2NH 3 ) has emerged as a promising, environmentally benign, and sustainable technology for N 2 fixation due to its mild operation conditions, utilizing water as a clean raw material, and no greenhouse gas (CO 2 ) emission. 1,2,8Ideally, the best NRR process is characterized by a catalyst with a small activation energy (Ea) to N 2 but also having relatively weak adsorption energy (ΔE) for intermediate species.Nonetheless, the electrocatalytic NRR is far from being used widely in practical applications owing to the fact that most of the currently used electrocatalysts display poor NRR kinetics and low faradaic efficiency (FE). 2,9The unfavorable N 2 adsorption and activation, as well as the parasitic hydrogen evolution reaction (HER) on the electrocatalysts, largely contribute to the impractical large-scale application of the NRR process. 1,9,10As a consequence, highly efficient electrocatalysts that can accelerate NRR kinetics and promote better faradaic efficiency are pursued.
Recently, various noble metal electrocatalysts (Au, Ag, Ru, Rh) have been used for electrochemical N 2 fixation; however, their low abundance and high cost hinder their widespread usage in electrocatalytic NRR processes. 11,12−15 Among these materials, the low-cost 2D molybdenum disulfide (MoS 2 ) nanostructures have been pursued as potential NRR catalysts owing to their nontoxicity, high chemical stability in liquid media, and poor activity for competitive hydrogen evolution reaction (HER). 14,16,17−25 Among the various 2D materials, graphene (Gr) or its derivatives is the most popular material to be used as a scaffold for MoS 2 nanosheets due to its large surface area, superior electrical conductivity, high charge mobility, and intrinsic flexibility. 26,27or instance, Wu et al. showed that the MoS 2 nanosheetreduced graphene oxide hybrid (MoS 2 -rGO) electrocatalyst achieved an FE of 4.58% and a high NH 3 yield rate of 24.82 μg h −1 mg cat .−1 as well as effectively suppressed HER. 23Despite the low FE, they attributed the NRR performance to abundant high-speed electron transport channels and the synchronously induced electronic coupling effect.As such, this study aims at increasing the NRR electrocatalyst performance by shifting the chemical equilibrium for HER by decreasing the concentration of the proton donor species (H 2 O or H 3 O + ), through an increase in electrolyte pH, while also focusing on the contribution of the MoS 2 phases on the NRR activity.

EXPERIMENTAL SECTION
2.1.Synthesis of MoS 2 @rGO Nanostructures.The MoS 2 @ rGO composite nanostructures were synthesized by using the hydrothermal technique.Initially, the reduced graphene oxide (rGO) nanosheets were synthesized via the microwave-assisted reduction procedure as previously reported. 28Typically, graphene oxide powder was irradiated with 1 kW inside argon (Ar) plasma for 3 min at a pressure of 10 mbar in a quartz flask (1 L volume) connected to a vacuum pump and an argon mass flow meter (200 mL/min), followed by cooling to room temperature under Ar gas, washing the rGO samples with a solvent, and finally drying overnight at 80 °C.For the anchoring of MoS 2 nanostructures, ∼140 mg of rGO nanosheets was dispersed in 35 mL of deionized water through ultrasonication for 1 h.Subsequently, 540 mg of ammonium molybdate tetrahydrate ((NH 4 ) 6 Mo 7 O 24 •4H 4 O, 99.99% Sigma) and 1.14 g of thiourea (CH 4 N 2 S, 99.8%, Sigma), as molybdenum (Mo) and sulfur (S) precursors, 29 were stirred into the rGO dispersion for 45 min, following which the dispersion was transferred to a 100 mL Teflonlined autoclave and then heated at 220 °C for 24 h (Figure S1).Likewise, the free-standing MoS 2 nanostructures were synthesized by using the same hydrothermal procedure.After cooling to room temperature, the black precipitate was thoroughly washed with ethanol and distilled water via centrifugation and then dried overnight at 70 °C.

Characterization of Active Materials.
The structural disorders were investigated using a Renishaw inVia Rama Microscope equipped with a laser excitation wavelength of 532 nm.Average I D /I G ratios were calculated from five measurements per sample.Surface functionalities of the samples were determined using a Nicolet iS50R FTIR spectrometer with a universal attenuated total reflectance (ATR) accessory.The morphological features of the samples were ascertained by using a Tescan Lyra dual beam scanning electron microscope (SEM) and the JEOLJ2010 transmission electron microscope (TEM).The thickness of the pristine MoS 2 and the 2D/2D MoS 2 @rGO flakes was determined using atomic force microscopy (AFM) on an Ntegra Spectra microscope (NT-MDT).Following the dispersion of the samples in DMF for 30 min via ultrasonication, the sample was drop-cast on a freshly cleaved mica substrate, and surface scans were performed in the tapping mode under ambient conditions.The measurements were acquired with a scan rate of 1 Hz and a scan line of 512 using cantilevers with a strain constant of 1.5 kN m −1 equipped with a standard silicon tip with a curvature radius lower than 10 nm.The surface area was determined from the N 2 adsorption and desorption isotherms by using a Micromeritics Tristar 3000 system.The specific surface area and the porosity were characterized using the Brunauer−Emmett−Teller (BET) method through adsorption−desorption measurements of N 2 and the pore distribution analysis at the freezing temperature of N 2 (77K).The crystallinity of the samples was analyzed using a Bruker D8 Advanced X-ray diffractometer (XRD, Bruker, Billerica, MASS, USA), equipped with a Cu-Kα radiation source.Finally, the surface chemical compositions of the samples were recorded on the ESCAProbeP X-ray photoelectron spectrometer (XPS) equipped with a monochromatic Al-α radiation source.Extended X-ray absorption fine structure (EXAFS) data were collected on the B18 beamline at the Diamond Light Source (DLS) 30 at the Mo K-edge.Data were acquired in transmission and fluorescence modes with a Si311 monochromator and Pt-coated mirrors.The pure MoS 2 and MoS 2 @rGO samples before the NRR were ground into a fine powder using a pestle and mortar and mixed with cellulose before being pressed into pellets and mounted on the sample holder.The exact masses of the samples used to create pellets of MoS 2 and MoS 2 @rGO before the NRR were 3.89 and 9.46 mg, respectively.Cellulose was added to reach a total pellet mass of ca.. 60 mg.The MoS 2 and MoS 2 @rGO after the NRR, given the nature and relatively low amount of samples available, were measured as foils sealed between two pieces of Kapton tape and mounted on a sample holder.EXAFS data were acquired in the 19800−21000 eV range corresponding to a k-range of up to 16 Å −1 with an energy resolution of 0.3 eV.EXAFS data analyses were performed with Athena software from the Demeter package. 31.3.Electrochemical Measurements.The electrochemical performance of the as-prepared nanocatalysts was recorded at room temperature on an Autolab PGSTAT 204 (Metrohm, Switzerland) potentiostat using a gastight H-type electrolytic three-electrode system (Figure S2).For both electrochemical (cyclic voltammetry (CV) and linear sweep voltammetry (LSV) at 100 mV s −1 scan rate) and NRR measurements, MoS 2 @rGO and/or MoS 2 nanocatalysts immobilized on the glassy carbon L-electrode (GCE, 5 mm in diameter) functioned as the working electrode, while the platinum (Pt) sheet and the Ag/AgCl electrode were the counter and reference electrodes, respectively, and 0.1 M KOH aqueous solution was the electrolyte.The modified GCE and the Ag/AgCl electrode, as the cathodic component, were separated from the counter electrode, the anodic component, by the pretreated Nafion 211 membrane. 32Prior to the measurements, the electrocatalyst ink was prepared by dispersing 4 mg of the nanocatalyst in 970 mL of absolute ethanol (EtOH, 99.8%, Merk) containing a 17 μL Nafion ionomer solution (5 wt %, Aldrich) by ultrasonicating for 1 h. 29Afterward, 10 μL of catalyst ink was drop-cast on the 0.1 mm Al 2 O 3 slurry precleaned GCE surface and dried under ambient conditions to form the working electrode.Finally, the cathodic and anodic components were continuously purged with high-purity N 2 (99.999%) at a constant flow rate of 20 mL min −1 , from which the ammonia (NH 3 ) and the byproduct hydrazine (N 2 H 4 ), produced from NRR, were collected in the 30 mL acid trap (0.1 M H 2 SO 4 ).All potentials were referenced versus Ag/AgCl unless mentioned otherwise.
2.4.Quantification of Ammonia and Hydrazine.The amounts of the produced NH 3 and its N 2 H 4 byproduct from the NRR process were determined using the HI83300 multiparameter photometer (Hanna Instruments, Rhode Island).Using the ASTM D1426 Nessler method 33,34 for determination of the concentration of NH 3 , 1 mL of the unreacted sample was pipetted into the 10 mL cuvette, following which 9 mL of ammonia high-range reagent B was used to bring the cuvette contents to the mark.After obtaining the zero background, four drops of ammonia high-range reagent A were added to the cuvette and mixed thoroughly, and after awaiting the color development, absorbance of the sample was measured at 420 nm.The yield rate of ammonia (Y.R. NH3 ) was estimated according to eq 1 where C NHd 3 is the concentration reading from the photometer in mg• L −1 , V is the volume of the acid trap in mL, t is the reduction time in seconds, and A is the area of the GCE in cm 2 .Ultimately, the faradaic efficiency (FE) of the NRR process was calculated based on eq 2 where C NHd 3 and V are described above, F is the faradaic constant, Mw NHd 3 is the molecular weight of ammonia (17 g mol −1 ), and Q is the accumulated charge of the electrode during the NRR process.
For quantification of the hydrazine concentration after the NRR process, the D1385 p-dimethylamino-benzaldehyde method was used.Typically, two 10 mL cuvettes were filled with the unreacted sample and deionized water, respectively.To each cuvette, 12 drops of the hydrazine reagent were added, after which the cuvette containing the deionized water was used for zero background correction, and the sample containing the hydrazine reagent was used for measurement of absorbance at 466 nm.Similar equations were used to determine the amount of the byproduct N 2 H 4 , whereby C Nd 2 Hd 4 was the concentration reading from the photometer in μg•L −1 and Mw Nd 2 Hd 4 was the molecular weight of hydrazine (32 g mol −1 ).

Morphology and Adsorption Properties.
The morphology of the composite nanocatalysts was investigated by using scanning electron microscopy (SEM), high-resolution transmission electron microscopy (HRTEM), and atomic force microscopy (AFM) characterization techniques.The HRTEM images showed thin nanosheets with a well-defined honeycomb atomic arrangement of MoS 2 embedded within the semicrystalline sea of rGO (Figure 1a), thereby confirming the crystallinity of the MoS 2 samples, as later highlighted by the XRD patterns.The SEM micrographs of the MoS 2 @rGO samples showed irregular nanosheets of MoS 2 decorating the wrinkled sheets of rGO, both at the edges and in between the rGO nanosheets, somehow depicting a 3D-like nanostructure (Figure 1b).The observed 3D-like morphology could be essential for maximizing accessibility to active sites during the NRR electrocatalytic reactions, essentially leading to improved catalytic activity.On the other hand, the pristine MoS 2 nanostructures revealed an agglomeration of well-defined MoS 2 nanosheets into flower-like nanoclusters (Figure S3a).The absence of the nanoclusters on the MoS 2 @rGO hybrid samples is suggestive of the abundance of catalytically active sites on the rGO nanosheets, thus providing sufficient anchoring sites for the attachment and growth of MoS 2 nanosheets.Thickness estimation of the MoS 2 and MoS 2 / rGO flakes was conducted via AFM.As shown in figure S4c, the pure MoS 2 flakes have an average thickness between 10 and 15 nm, whereas upon coupling with rGO, the thickness increases to 25−30 nm (Figure S4a).Interestingly, the heterojunction nanostructures tend to form agglomerated clusters (Figure S4b), which could be ascribed to the strong interface interactions between the two 2D structures.
Given the 3D-like network of the MoS 2 @rGO nanocatalysts, a type IV N 2 adsorption/desorption isotherm with an H3 hysteresis loop (Figure S5) was evident at a relative pressure P S /P 0 of 0.5−0.9. 35The observed isotherms corresponded to the formation of nonporous materials containing an assemblage of narrow slit-like pores due to the planar morphology of MoS 2 and rGO nanosheets. 36Ultimately, the BET specific surface area (SSA) of MoS 2 @rGO was determined to be 56.1 m 2 g −1 (Table S1), which was slightly larger than that of the MoS 2 nanoclusters (45.12 m 2 g −1 ).The high SSA for MoS 2 @ rGO hybrids could subsequently be viewed as a crucial parameter for providing more catalytically active sites for NRR, in so doing facilitating the rapid production of ammonia.
The broad diffraction peaks at ∼23 and ∼41°correspond to the (002) and (101) lattice planes of disordered carbon, respectively. 39However, the absence of the diffraction planes of rGO for the MoS 2 @rGO sample can be attributed to the low content of the carbonaceous component in the hybrid nanocatalysts.More importantly, the distinction between the types of MoS 2 in both samples, viz.octahedral 1T-MoS 2 and hexagonal 2H-MoS 2 , was unclear given that there was no sign of diffraction patterns associated with 1T-MoS 2 due to the fact that most XRD peaks of 1T-MoS 2 overlap with those of 2H-MoS 2. 37 As such, the samples were further analyzed using Raman spectroscopy.
In order to determine the nature of the MoS 2 anchored on the reduced graphene oxide nanostructures, Raman spectroscopy measurements were carried out as shown in Figure 2b.The spectra of the MoS 2 @rGO nanocatalysts displayed two characteristic first-order Raman bands corresponding to the E 2g 1 (∼381.4−43 In addition to the 2H-MoS 2 , two minor vibrational modes at ∼215.7 and ∼337.9 cm −1 corresponded to the phonon modes in 1T-MoS 2 , thus indicating that the free-standing MoS 2 nanoclusters comprise a mixture of octahedral and planar nanostructures. 40,41On the contrary, due to the strong interface interactions as well as structural compatibility between MoS 2 and rGO, 20,23,25 the results showed that predominantly hexagonal MoS 2 nanosheets are effectively anchored on the large area surface of rGO.
In addition to the MoS 2 vibrational modes, the Raman spectra of the MoS 2 @rGO nanocatalysts also exhibited two first-order Raman peaks for the carbonaceous materials, located at ∼1346 cm −1 (D band) and ∼1588 cm −1 (G band), indicating the presence of reduced graphene oxide in the composite nanocatalysts. 44,45The defect-level indicator was determined from the estimated ratio of the integrated area under the D band to that of the G band (vis.I D /I G ).Given the relatively high defect density ratio (∼3.17), a better NRR performance can be anticipated from these unique defect-rich MoS 2 @rGO nanocatalysts due to the combined contribution from the few-layered MoS 2 nanosheets, the highly defective rGO supports, and the synergetic effect of the two 2D structures.The surface functionalities on the MoS 2 @rGO samples were determined from Fourier transform infrared spectroscopy (FTIR, Figure S6).It is noteworthy to highlight that the absorption region between ∼1900 and 2100 cm −1 is associated with the modes from the diamond crystal on the instrument.Lack of a strong and broad absorption band at ∼3300−3500 cm −1 , arising from the stretching vibrations of the OH bonds of the hydroxyl and carbonyl groups, confirmed the successful reduction of GO into rGO upon microwave irradiation as well as during the hydrothermal anchoring process. 46Additionally, the MoS 2 @rGO nanocatalyst spectrum exhibited peaks at ∼1303, ∼1630, and ∼2600 cm −1 , corresponding to the C−OH carbonyl, C�C in-plane, and C−H alkyl stretching bands in rGO. 46Furthermore, a minor peak at ∼10 66 cm −1 represented the S−OH asymmetrical stretching modes, suggesting minor surface functionalization of rGO with sulfophenyl groups during the hydrothermal process. 47For the pristine MoS 2 sample, the vibrational bands at ∼1066, ∼961, ∼835, and ∼631 cm −1 showed the formation of MoS 2 nanostructures. 48,49.3.Surface Composition Determination.The chemical valence states of the elements in the nanocatalysts as well as the surface compositions were determined by using X-ray photoelectron spectroscopy (XPS).The survey spectra (Figure S7) of the MoS 2 @rGO and MoS 2 samples revealed the presence of Mo 3d (∼229 eV), S 2p (∼162 eV), C 1s (∼284 eV), N 1s (∼395 eV), and O 1s (∼532 eV).The S/Mo element ratio of the hybrid nanocatalysts was estimated from the integral peak area of the XPS survey spectra and was determined to be ∼2.0.The carbon and nitrogen peaks could be ascribed to the contribution of the precursor salts.42,50 The presence of the 1T phase on the pristine MoS 2 sample was shown by additional component peaks at 228.2 and 231.2 eV (Figure S7b), corresponding to the Mo 4+ d 5/2 and Mo 4+ d 3/2 oxidation states of the 1T phase.42,50 The binding energies at 229.1 and 232.3 eV corresponded to Mo 5+ d 5/2 and Mo 5+ d 3/2 regions.50,51 Compared to the pristine MoS 2 sample (Figure S7), the binding energies of Mo 4+ in the MoS 2 @rGO are slightly negatively shifted.The negative shift of the Mo 4+ species can be ascribed to the interaction of the Mo atoms with the graphene lattice.The existence of the fifth XPS peak at 235.7 eV can be indexed to Mo 6+ 3d 5/2 , indicating regions of surface oxidation of MoS 2 .50,52 Last but not least, the core-level spectrum of the S 2p peak (Figure 3b) was resolved into a doublet corresponding to S 2p 3/2 (∼161.4eV) and S 2p 1/2 (∼163.9eV), respectively.This represents the presence of bridging S 2 2− -type species bonded to Mo 4+ oxidation states.50,52 Ambiguously, the component peak at ∼163.9 eV can be assigned to C−S−C bonding and indicates the introduction of S atoms into the defective graphene lattice.53,54 The C 1s spectrum for the MoS 2 @rGO sample (Figure 3c), was fit with four components centered at 284.1, 285.4,286.8, and 288.3 eV corresponding to the presence of graphitic sp 2 C�C bonds, defect-induced C−C bonds, and the oxygenated carbon bonds, respectively.55,56 Finally, the O 1s spectra (Figures 3d and S7d

Electrochemical Performance.
Electrochemical methods were performed to verify the conjecture for the NRR performance of the MoS 2 @rGO electrocatalyst within a potential window of −2 to 0 V. Verification of the voltammetric NRR activity was performed by plotting the linear sweep voltammetric (LSV) polarization curves in 0.1 M KOH electrolyte solution saturated with N 2 or Ar, as shown in Figure 4a.The reaction in argon-saturated electrolyte was performed so as to recognize the possible contributions to nitrogen production from ubiquitous N-containing contaminants such as adventitious NH 3 in N 2 gas supply, the residues from the Mo-precursor salt, or the electrolyte.A slightly higher cathodic current density (Figure 4a, inset) is evident within the sweeping potential range of −1.20 to 1.40 V during purging with N 2 as compared to Ar purging.From the determination of the NRR performance after 2 h reaction time in an alkaline medium, the contribution to N 2 production from potential contaminants could be considered to be practically minimal, as  indicated by the NH 3 yield rate of 0.054 mg h −1 cm −2 and 0.06% FE under Ar gas purging.To explore the reaction kinetics of the NRR process catalyzed by the MoS 2 @rGO electrocatalysts, the polarization curves as a function of reaction time (j−t) at different cathodic potentials were plotted as shown in Figure 4b.From the chronoamperometric experiments at different cathodic potentials (Figure 4c), a low NRR sensitivity (FE) of 2.98% at an NH 3 yield rate of 1.48 ± 0.07 mg h −1 cm −2 was recorded for the −1.20 V sweeping cathodic potential.A gradual increase of the potential to −1.30 V saw a considerable NRR improvement (FE ≈ 34.7%) with a remarkable NH 3 yield rate of 3.98 ± 0.19 mg h −1 cm −2 .However, an increase to more negative potentials (−1.40 V) showed a gradual loss of the NRR activity (FE ≈ 2.24%) and a consequently decreasing NH 3 yield rate (2.0 ± 0.1 mg h −1 cm −2 ), thereby indicating potential increased dominance of the competitive HER process.Evidence of varying amounts of ammonia production at different cathodic potentials was revealed by the varying color intensities of the solutions (Figure 4d) after the Nessler method.The obtained NRR activity results showed the importance of the formation of hybrid structures for enhancing the NRR performance catalyzed by the MoS 2 @rGO nanocatalysts in comparison to the NRR process catalyzed by pristine MoS 2 nanocatalysts (Figure S3a).In particular, MoS 2 nanocatalysts led to a very low NRR sensitivity (FE) of 0.23% at an NH 3 yield rate of 26.8 ± 0.07 μg h −1 per square area of the covered GCE surface at the overpotential of −1.20 V.Besides the recorded low NRR sensitivity on the MoS 2 nanocatalysts (Figure S8a), due to the increased dominance of the competitive HER process within the sweeping potential window, the results also revealed poor selectivity of the MoS 2 nanocatalysts for ammonia production to that of the hydrazine byproduct in comparison to the MoS 2 @rGO nanocatalysts (Figure S8b).In comparison with the reported hybrid catalysts of MoS 2 and graphene derivatives or 2D structures in either acidic or neutral media (Table 1), the results revealed that the MoS 2 @rGO heterojunction nanocatalysts exhibited better electrocatalysis for the NRR in the alkaline media.Durability studies constitute another key factor for evaluating a high-performing NRR catalyst for practical application, and this is typically measured by chronoamperometric analysis.As shown in Figure 5a, the MoS 2 @rGO nanocatalysts maintain a relatively stable NRR catalytic activity after multiple cycles of operation at −1.30 V.After ∼8 h of operation, 4 cycles with 2 h intervals, a steady-state production of NH 3 was observed, retaining ∼88% of the sensitivity (FE).The outstanding durability could be attributed to the structural properties of the MoS 2 @rGO nanocatalysts, which subsequently guaranteed an enhanced NRR catalytic activity.For instance, regardless of the slight aggregation of crystalline MoS 2 (size ∼2.8 ± 0.8 nm) within the rGO mass after the 2 h NRR experiments (Figure 5b−d), the preservation of the 2H phase of MoS 2 assisted in retaining the nitrogen reduction activity.However, this aggregation could be presumed to be the cause for the drastic loss of activity with prolonged use of the MoS 2 @rGO nanocatalysts, as observed by only 20% FE after the fifth cycle of NRR experiments.Nonetheless, the thin morphology of the MoS 2 nanosheets, despite their clustering, still provided a large surface area and a slightly mesoporous structure, thereby facilitating a rapid electron−ion transport pathway as well as abundant channels for the transport of reactant molecules.Moreover, the defect-rich rGO nanosheets provided sufficient active sites for the easy adsorption and dissociation of N 2 , whereas the interaction of Mo atoms with the graphene lattice enhanced the electrical conductivity (R s ≈ 16.9 Ω), consequently ensuring faster electron transfer and low charge transfer resistance (R ct1 ≈ 74.3 Ω and R ct2 ≈ 30.4 Ω), as shown by the electrochemical impedance spectroscopy (EIS) Nyquist plots (Figure S9d and Table S4).
To ascertain the obtained NRR performance and sensitivity of the MoS 2 @rGO hybrid nanocatalysts in alkaline media, hydrogen evolution reaction (HER) activity of each electrocatalyst was established via plotting of the LSV curves in 1.0 M KOH electrolyte (Figure S9a).As observed in Figure 5c, MoS 2 @rGO nanocatalysts exhibit an HER Tafel slope of ∼214 mV dec −1 , while the MoS 2 nanocatalysts revealed a Tafel slope of ∼188 mV dec −1 .The slow HER kinetics on the MoS 2 @rGO nanocatalysts could be attributed to the predominance of the 2H-MoS 2 phase, thereby inhibiting the rate of HER due to the fact that the H + /H 2 redox potentials lie above the conduction band energy level of the 2H-MoS 2 phase, 41,57−59 whereas in comparison to the pristine MoS 2 , the HER activity could primarily be slightly enhanced by the improved accessibility to the active sites.Based on these HER activity measurements, it is evident that engineering the form of MoS 2 anchored on rGO nanosheets plays a pivotal role in promoting NRR, such that it becomes more competitive against HER.
3.5.N 2 Adsorption and Reduction Mechanism.In order to shed light onto the structural and electronic differences between the samples that could lead to insights into the mechanism of the enhanced NRR activity observed for MoS 2 @rGO, we performed ex situ EXAFS and XANES measurements at the B18 beamline at the Diamond Light Source.The samples in question were MoS 2 and MoS 2 @rGO before and after NRR.The four XANES spectra of the samples at the Mo K-edge, provided in Figure 6a of the Supporting Information, do not show any significant change before and after the reaction or in the presence (or absence) of the rGO substrate.This evidence suggests that the Mo p states do not contribute substantially to the reaction mechanism.Furthermore, in Figure 6b, we also report the Fourier-transformed EXAFS data, which show minimal variation in the intensities of the peaks.On this basis, we assert that there are minimal changes to the local structure and coordination geometry around the Mo atoms in all of the different samples.Notwithstanding the information acquired by XANES and EXAFS characterization, a detailed formulation of the mechanism behind the catalysis and associated changes upon interaction with the support remains an open question.It should be noted that X-ray absorption spectroscopy (XAS) captures an average over all of the Mo atoms in the beam; if a small fraction of Mo atoms is responsible for the catalytic activity or is modified by the interaction with the rGO support, these changes may not be detectable by XAS.Therefore, given all of the information, we can postulate that electrons in other Mo-orbitals (4d shell), or the presence of low-concentrated Svacancies or rGO, are responsible for the enhanced NRR.

CONCLUSIONS
In summary, we have demonstrated the significance of catalyst design in achieving a highly active and selective NRR electrocatalyst through the formation of MoS 2 @rGO 2D/2D hybrid structures.The results indicated that the ultrathin layered structure, the relatively large surface area, and the defect-rich morphology of the MoS 2 @rGO 2D/2D hybrid electrocatalysts played an important role in the enhanced NRR performance, characterized by a high sensitivity (FE) of 34.7% with an NH 3 yield rate of 3.98 ± 0.19 mg h −1 cm −2 at the overpotential of −0.3 V vs RHE in 0.1 M KOH solution.Moreover, the semiconducting behavior of the 2H-MoS 2 phase further promoted the NRR process by providing additional active binding sites at S-vacancies while also inhibiting the rate of HER due to the difference in the energy levels between the H + /H 2 redox potentials and the conduction band.By tuning the phases on MoS 2 nanosheets, and thereby controlling their electrochemical properties, the study provided an inexpensive route for designing future 2D/2D hybrid electrocatalysts, which could also be useful for other energy conversion and storage systems that are typically limited by the HER process, such as carbon dioxide reduction reaction.

■ ASSOCIATED CONTENT Data Availability Statement
Main data presented in this study are available at https:// zenodo.org/records/,* sı Supporting Information The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acsami.4c00719.S1, textual properties of pristine MoS 2 and the MoS 2 @rGO hybrid samples; Table S2, Raman parameters for the pristine MoS 2 and the MoS 2 @rGO hybrid samples; Table S3, atomic compositions of the composite samples; and Table S4, EIS analysis of pristine MoS 2 and the MoS 2 @rGO hybrid samples post HER experiments (PDF) ■ AUTHOR INFORMATION
Figure 3 displays the peak fitted XPS spectra for the MoS 2 @rGO sample, while those of the pristine MoS 2 are shown in figure S7.The high-resolution Mo 3d spectrum (Figure 3a) consists of component peaks located at 228.6, 230.1, 231.8, 233.2, and 235.3 eV.The two main peaks at 228.6 and 231.8 eV are assigned to Mo 4+ d 5/2 and Mo 4+ d 3/2 of the 2H phase, while the component peak at a lower binding energy (225.8eV) is attributed to S 2s.
) showed peaks at 533.3, 532.5, 531.7, and 530.8 eV, representing the contribution from Mo−O and/or C−OH, C−O, and O�C bonding configurations.50

Figure 5 .
Figure 5. (a) Stability test of MoS 2 @rGO nanocatalysts (NH 3 yield rate and FE) after five-cycle 2 h NRR experiments.(b−d) HRTEM micrographs after the first cycle of NRR experiment.

Figure 6 .
Figure 6.(a) Normalized XANES for MoS 2 and MoS 2 @rGO samples before and after 3 h NRR measurements at the Mo K-edge.(b) EXAFS Fourier transform (FT) k 2 -weighted χ(R) signal for MoS 2 and MoS 2 @rGO samples before and after NRR at the Mo K-edge.The k-range used for FT is 2.5−14.1 Å −1 .

Figure S1 ,
FigureS1, synthesis procedure for anchoring MoS 2 nanosheets on rGO nanosheets; FigureS2, representation of the H-type electrolytic cell; FigureS3, SEM, lowmagnification TEM, and HRTEM micrographs of the pristine MoS 2 samples, as well as SEM EDX mapping profiles; FigureS4, AFM images and height profiles for the MoS 2 @rGO hybrid, as well as pristine MoS 2 samples; FigureS5, adsorption isotherms of pristine MoS 2 and the MoS 2 @rGO hybrid samples; FigureS6, FTIR spectra of pristine MoS 2 and the MoS 2 @rGO hybrid samples; FigureS7, XPS survey spectra of MoS 2 and MoS 2 @rGO, as well as fitted Mo 3d + S 2s, S 2p, and O 1s spectra for MoS 2 nanostructures; FigureS8, NRR performance of MoS 2 nanocatalysts (NH 3 yield rate and FE) versus cathodic potential, yield rate of hydrazine (N 2 H 4 ) after NRR experiments catalyzed by MoS 2 and MoS 2 @rGO nanocatalysts, polarization curves in N 2 and Ar, as well as chronoamperometric curves at various potentials of MoS 2 nanocatalysts in N 2saturated 0.1 M KOH electrolyte; FigureS9, polarization curves, overpotentials, and Tafel plots, as well as Nyquist plots of the electrocatalysts during HER activity in 1.0 M KOH; TableS1, textual properties of pristine MoS 2 and the MoS 2 @rGO hybrid samples; TableS2, Raman parameters for the pristine MoS 2 and the MoS 2 @rGO hybrid samples; TableS3, atomic compositions of the composite samples; and TableS4, EIS analysis of pristine MoS 2 and the MoS 2 @rGO hybrid samples post HER experiments (PDF)